The subject matter of this invention relates generally to current limiting fuses and more particularly to current limiting fuses of the type having auxiliary arcing means.
Current limiting fuses have been an important part of the protection of high voltage power systems for many years. Generally high voltage current limiting fuses include central mandrels or spiders around which an elongated fuse element is wound to compress the length of the fuse body without actually reducing the length of the fuse element. Generally, the fuse element or ribbon has cut therein notches of reduced cross-section at which arcing occurs during a fusing operation. The multiple arcs formed at the notches each contribute an increment of voltage for current limiting purposes. It has been found that in those fuses which have relatively high cross-sectional area for carrying relatively high values of normal load current, it is difficult to achieve current interruption at a relatively low value of overload current. One way for correcting this is taught in U.S. Pat. No. 3,243,552, issued Mar. 29, 1966 to H. W. Mikulecky entitled "Current Limiting Fuse". In this case, an auxiliary fuse element which generally parallels the main fuse element but which is not normally connected thereto is positioned at the ends thereof in close proximity to the main fuse element. When the main fuse element melts during a relatively low overload condition, melting occurs in the center section of the element. A potential difference thus is created between the ends of the auxiliary fuse elements and the main element thus producing electrical arcs in these later regions. This arcing eventually burns through and severs the main element in these regions. These arcs increase the number of electrical arcs which oppose the overload current that is being interrupted. A problem associated with the aforementioned scheme lies in the fact that the arcing and burning process tends to be difficult to control. Generally, clip-on arcing contacts are utilized at the ends of the auxiliary fuse element and spaced relatively close to the main fuse element. The distance between the arcing contact and the main element is often very critical and tolerances must be very close. The density of the arc quenching sand which may exist between the main fuse element and the arcing contacts can be very determinative of the breakdown voltage required. It can vary under certain conditions. U.S. Pat. No. 3,983,526 issued Sep. 28, 1976 to Koch and entitled "Current Limiting Fuse With Auxiliary Element Arcing Clip Spaced By Nonporous Dielectric Member" attempts to solve the aforementioned problem. With the teachings of the latter-mentioned patent, apparatus is utilized for maintaining the arcing contact in the main element at a specific dimension. However, this leads to difficulty in the assembly process. Furthermore, it can be seen that the time required for the arc from the auxiliary element to burn through the main element varies considerably depending upon whether the main element passes over the arcing clip at a solid area, a perforated area, or somewhere therebetween. On a high voltage fuse utilizing several feet of main fuse element, it is very difficult to wind the element so that the aforementioned variation is accounted for. To overcome this, U.S. Pat. No. 3,983,524 issued Sep. 28, 1976 to Koch entitled "Electrical Current Limiting Fuse Having Fusible Element With Additional Cross-Sectional Necks At An Arcing Clip" teaches the use of extra perforations in the area that passes over the arcing clips or terminals. This reduces the difficulty associated with utilizing the arcing clips but does not necessarily remove it. Furthermore, the utilization of extra areas of reduced cross-section add to the complexity of the manufacturing process. It would be advantageous therefore, if a current limiting fuse which utilizes an auxiliary fuse element to cause multiple arcing on low current faults but which does not suffer from the above-mentioned problems could be found.